This invention relates to in situ recovery of shale oil, and more particularly to a mining system for excavation and explosive expansion of oil shale formation in preparation for forming an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is call "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooled oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
It is desirable to form a fragmented mass having a reasonably uniformly distributed void fraction, i.e., a fragmented mass of reasonably uniform permeability, so that retorting gas can flow generally uniformly through the fragmented mass during retorting operations. Techniques used for excavating void spaces in a retorting site and for explosively expanding formation toward the voids can affect the uniformity of particle size or permeability of the fragmented mass. A fragmented mass having reasonably uniform permeability in horizontal planes across the fragmented mass can avoid bypassing portions of the fragmented mass by retorting gas, which can otherwise occur if there is gas channeling through the fragmented mass owing to non-uniform permeability.
It is desirable that techniques used in excavating and explosively expanding formation within an in situ oil shale retort site provide a means for controlling the void fraction distribution within a fragmented mass being formed so that a reasonably uniformly distributed void fraction can be provided in the resulting fragmented mass.
U.S. Pat. No. 4,043,597 discloses a method for forming a fragmented mass, in which a zone of unfragmented formation within a retort site can be explosively expanded in lifts, i.e., in separate horizontal layers with a time delay between expansion of each layer. If expansion of such lifts is not carefully controlled, non-uniform void fraction distribution within the fragmented mass can result. This can lead to channeling of gas flow through the fragmented mass during subsequent retorting operations. The mining system of this invention facilitates control over the void fraction distribution in a fragmented mass being formed.
U.S. Pat. Nos. 4,043,597 and 4,043,598 disclose methods for forming a fragmented mass in a horizontal free face system with intermediate level voids. The mining and construction costs involved in preparing a retorting site for explosive expansion can be reduced by eliminating excavation of multiple voids and corresponding retort level access drifts at intermediate levels of a retort site. Elimination of such intermediate void spaces from a retort can avoid the presence of large unsupported areas within a retort site where workmen can be present during mining operations. The present invention facilitates use of a two-level, horizontal free face mining system in which a fragmented mass can be formed without excavating multiple void spaces and corresponding retort level access drifts at different intermediate levels within a retort site. In the present system the need for workers being present in the void spaces within the retort site is avoided.